As well known, an electrodeposited copper foil is produced by a method of using an aqueous solution of sulfuric acid-copper sulfate as an electrolyte, filling the electrolyte between an insoluble anode of titanium coated with a platinum metal element or its oxide element and a titanium cathode drum disposed facing the anode, conducting a DC current between both electrodes while a cathode drum is rotated at a constant rate to precipitate copper on a cathode drum surface, peeling precipitated copper from a cathode drum surface, and continuously winding this.
In the present invention, a surface of a side on which an electrodeposited copper foil has been contacted with a cathode drum surface is referred to as “shiny side”, and a reverse surface is referred to as “matte side”.
The electrodeposited copper foil is produced as described above is called as “electrodeposited copper foil without treatment” by a person skilled in the art, and this is not used as this electrodeposited copper foil without treatment and, when an electrodeposited copper foil for a printing circuit is obtained, a roughening treatment step for the purpose of improving adherability with an insulating resin, and various surface treatment steps for the purpose of imparting heat resistance, chemical resistance and rust proofness are performed.
In old times, for the purpose of sharpening (roughening) a crest-root shape of a matte side and suppressing a pinhole, 10 to 100 mg/L of chlorine ion and 0.1 to 4.0 mg/L glue or gelatin have been added to an electrolyte in a step of producing an electrodeposited copper foil without treatment (see Table 1, Comparative Example 6 of Table 2, and FIG. 3, infra). However, in recent years, in a printed circuit board and a lithium ion secondary cell anode current collecting body which are utility of an electrodeposited copper foil, a thin electrodeposited copper foil having as low roughness as possible on a matte side, a small roughness difference between a shiny side and a matte side (since a shiny side copies a smooth shape of a cathode drum surface, a roughness difference necessarily occurs between a shiny side and a matte side), and an excellent elongation rate at a high temperature has been demanded.
This is because, in a printed circuit board, since a path difference between a signal flowing on a shiny side and a signal flowing on a matte side can be reduced by reducing roughness of a matte side upon occurrence of skin effect by speedup of a signal frequency, the effect of preventing delay of a signal and, at the same time, retaining a resistance between insulating layers in a multi-layered substrate is obtained and, by improving an elongation rate at a high temperature, the effect of following resin flow generated at lamination of a multi-layered substrate, and retaining connection reliance in a throughhole is obtained and, further, by thinning a foil, the effect of improving a circuit precision accompanied with finer line and fine patterning is obtained.
On the other hand, in a lithium ion secondary cell cathode current collecting body, by reducing roughness of a matte side, a difference in a surface area between a shiny side and a matte side can be reduced and, accompanying this, the effect of decreasing necessary considering of a difference in a cell reaction is obtained.
However, it is difficult to reduce a roughness difference between a shiny side and a matte side, and satisfy practical various mechanical properties.
Previously, in a process for producing an electrodeposited copper foil, it has been known that a roughness difference between a shiny side and a matte side can be reduced by appropriately selecting and adding various water-soluble polymer materials, various surfactants, various organic sulfur-based compounds, and a chlorine ion to an electrolyte. For example, Japanese Patent No. 3313277 discloses a process for producing an electrodeposited copper foil using an electrolyte to which a compound having a mercapto group, a chloride ion, as well as a low-molecular glue having a molecular weight of 10000 or smaller and a polymer polysaccharide have been added (page 1), and JP-A-2002-506484 discloses a process for producing an electrodeposited copper foil using an electrolyte to which low-molecular water-soluble cellulose ether such as hydroxyethylcellulose, low-molecular water-soluble polyalkylene glycol ether such as polyethylene glycol, low-molecular water-soluble polyethyleneimine and a water-soluble sulfonated organic sulfur compound have been added (page 2, pp. 18-22).
The present inventors performed many experiments for obtaining an electrodeposited copper foil by the processes for producing an electrodeposited copper foil disclosed in the above respective gazettes, and investigated various properties of the resulting electrodeposited copper foils, and found that, regarding an electrodeposited copper foil obtained by the process for producing an electrodeposited copper foil disclosed in Japanese Patent No. 3313277, a tensile strength at 25° C. measured within 20 minutes from completion of electrodeposition exhibits a high value of 800 MPa, but the tensile strength is reduced with time, and a tensile strength at 25° C. measured at 240 minutes after completion of electrodeposition is reduced from 800 MPa to around 350 MPa. In addition, it was also found out that the tensile strength is also reduced by heat treatment, and a tensile strength at 25° C. measured after heat treatment at 100° C. for 10 minutes is reduced from 800 MPa to 320 MPa (see Table 1, Comparative Example 7 of Table 2, and FIG. 1, infra).
This is thought as follows: in the case of the process for producing an electrodeposited copper foil disclosed in Japanese Patent No. 3313277, since a copper foil precipitated on a cathode drum is composed of small crystal particles, the small crystal particles tend to migrate to the thermodynamically stable state by minimizing a surface area at room temperature, primary recrystallization occurs at room temperature driven by interface energy at a crystal grain boundary, crystal particles become coarse, and remarkable reduction in a tensile strength occurs.
There is a problem that, when a tensile strength of an electrodeposited copper foil is reduced, a crease is easily formed upon thinning a foil, and handling property at lamination of a multi-layered substrate is deteriorated.
In addition, in the case of the process for producing an electrodeposited copper foil disclosed in JP-A-2002-506484, since a concentration of respective additives relative to an electrolyte is comparatively high, copper powders are precipitated at electrolysis, causing so-called scorching state, and an electrodeposited copper foil cannot be peeled from a cathode drum surface (see Table 1, Comparative Examples 8 and 9 of Table 2, infra).
Accordingly, a technical object of the present invention is to provide an electrodeposited copper foil with low surface roughness having an extremely low lowering rate of a tensile strength with time or accompanied with heat treatment, and having an excellent elongation rate at a high temperature, specifically, an electrodeposited copper foil with low roughness having a matte side roughness Rz of an electrodeposited copper foil of 2.5 μm or lower, a tensile strength at 25° C. measured within 20 minutes after completion of electrodeposition of 500 MPa or higher and, at the same time, a lowering rate of a tensile strength at 25° C. measured at 300 minutes after completion of electrodeposition of 10% or lower, or a lowering rate of a tensile strength at 25° C. measured after heat treatment at 100° C. for 10 minutes, and an elongation rate at 180° C. of 6% or higher.
The present inventors continued to intensively study in order to attain the aforementioned object and, as a result, obtained notable findings that, by the presence of five additives of hydroxyethylcellulose, polyethyleneimine, acetylene glycol, a sulfonate salt of an active organic sulfur compound, and a chlorine ion in an electrolyte comprising an aqueous solution of sulfuric acid-copper sulfate, an electrodeposited copper foil having a matte side roughness Rz of an electrodeposited copper foil of 2.5 μm or lower, a tensile strength at 25° C. measured within 20 minutes from completion of electrodeposition of 500 MPa or higher and, at the same time, a lowering rate of a tensile strength at 25° C. measured at 300 minutes after completion of electrodeposition of 10% or lower, or a lowering rate of a tensile strength at 25° C. measured after heat treatment at 100° C. for 10 minutes of 10% or lower, and an elongation rate at 180° C. of 6% or higher is obtained, accomplishing the aforementioned object.